It is generally known that transformers are used in energy distribution networks in order to couple power supply units having different voltage levels. Oil-immersed transformers are usually used at relatively high voltage levels, such as, for example, 110 kV/380 kV, whereas, at voltage levels lower than this, for example, 30 kV, 72.5 kV and, occasionally even 110 kV, dry transformers or gas-filled transformers are common.
Oil-immersed transformers often comprise so-called load switches, by means of which, under load current, it is possible to switch between different taps of the transformer winding, and so the voltage in the network can be regulated within wide limits by adjusting the transformation ratio of the transformer. A load switch typically comprises multiple connection contacts, which are arranged along a track and are connected to different taps of a transformer winding. A main contact is mechanically moveable along the track and, as a result, can be alternatively connected to one of the taps. The taps are usually arranged along a circular track, and so the desired taps are contacted by means of a rotary movement of the main contact.
In the case of oil-immersed transformers, load switches of this kind are located in an oil-filled region of the transformer tank which, for the sake of cleanliness, is separated from the region in which the transformer is disposed. Due to the arrangement of such a load interruptor in oil, as the insulating medium, said load interruptor can be designed to be substantially smaller than is possible with an arrangement in air.
There is an increasing demand for a possibility for adjusting the transformation ratio of dry transformers under load for relatively low voltage levels as well, for the sake of improved controllability. In the case of dry transformers, the use of oil is intentionally dispensed with, however, and so a different design is required as compared to oil-immersed transformers, in particular with regard to cooling and insulation.
A disadvantage thereof, in the case of dry transformer load switches, is that the elimination of oil, as the insulation medium, results in a substantially increased risk of electric arcing and that a dry transformer load switch must be designed to be sufficiently large in order to reduce such a risk.